1. Field of the Invention
The present invention relates to an image signal processor for detecting and removing duplicate fields of an image signal such as an NTSC television signal of 30 Hz produced from a movie film of 24 Hz by means of a 3-2 pull-down system or the like.
2. Description of Related Art
While a movie is made of a film of 24 Hz, the NTSC(National Television System Committee) signaling system which is the television system presently adopted in Japan, the U.S. and other countries is an interlace signaling system having a frame rate of 30 Hz. Each NTSC signal frame is composed of two fields (hereinafter referred to respectively as a first field and a second field). Therefore, the 3-2 pull-down operation is implemented in order to convert a movie film signal into the NTSC image signal.
This point will be explained below with reference to FIG. 13. In FIG. 13, a movie film signal is shown in the upper stage and an NTSC image signal is shown in the lower stage. Fields 501 and 503 are generated as signals of the first field of the NTSC signal from one frame 401 of the movie film signal and Field 502 is generated as a signal of the second field. Regarding the array of like fields, the same signal is repeated in Fields 501 and 503 of the first field of the NTSC signal and the same signal is repeated in Fields 506 and 508 of the second field as shown in the figure. Thus, the odd frame of the movie film signal is displayed for a period of three fields and the even frame is displayed for a period of two fields on the converted NTSC signal. The signal of 24 Hz is converted into the signal of 30 Hz by making five frames of the NTSC signal from four frames of the movie film signal. The converted signal is called a telecinema signal.
Here, when an amount of information of the image signal is to be compressed by using an image coding system such as MPEG-2(Moving Picture Experts Group) standardized by the International Standardization Organization for example, there is a possibility that the coding efficiency cannot be enhanced when the telecinema signal is coded because redundant duplicate fields are also coded. Thus, there has been implemented a process of implementing the coding process after removing the duplicate fields and converting the 30 Hz signal into a signal of 24 Hz which corresponds to the movie film signal. Then, the 3:2 pull-down operation is implemented after decoding the converted 24 Hz signal to form an NTSC signal of 30 Hz again.
FIG. 14 is a diagram showing a structure of a prior art television signal converter disclosed in Japanese Patent Laid-Open No. Hei. 9-55879 for example. The converter comprises VF generating means 1 for generating a VF signal indicative of whether an input signal belongs to the first field or the second field, a duplicate field detecting means 2 for detecting a duplicate field, and a duplicate field removing means 3.
FIG. 15 is a diagram showing a structure of the duplicate field detecting means 2 in detail. The duplicate field detecting means 2 comprises field memories 10 and 11, a subtracter 12, absolutizing means 13, one-field cumulative adder means 14, duplicate field determining means 15, comparator means 16, a memory 17 for storing a cumulative addition signal for a period of one field, and threshold value determining means 18.
The operation of the converter will now be explained. In FIG. 14, the VF generating means 1 decides whether an input image signal 100 is a signal belonging to the first field or the second field and outputs the VF signal 102. Then, the duplicate field detecting means 2 detects a duplicate field and outputs a duplicate field discriminating signal 103. The duplicate field removing means 3 removes the duplicate field of an image signal 101 which has been delayed for a period of two fields by the duplicate field detecting means 2 in correspondence to the duplicate field discriminating signal 103 and outputs a television signal 104 of 24 Hz.
Here, the operation of the duplicate field detecting means 2 will be explained in detail with reference to FIG. 15. The image signal 100 is input to the field memory 10 and to the subtracter 12. The subtracter 12 takes a difference between the image signal 101 which has been delayed by a period of two fields by the two field memories and the input image signal 100 and outputs a difference signal 201.
The absolutizing means 13 finds an absolute value of the difference signal 201 and outputs an absolute difference signal 202. The one-field cumulative adder means 14 cumulatively adds the absolute difference signals 202 of only one field and outputs a cumulative addition signal 203. The cumulative addition signals 203 corresponding to the first and second fields are output alternately. The memory 17 holds the cumulative addition signals 203 sequentially and outputs a result of the cumulative addition of the first field and a result of the cumulative addition of the second field in correspondence to the VF signal 102 so that cumulative addition signals of the first and second fields can be compared by the comparator means 16. The comparator means 16 decides whether the image signal 100 is a duplicate field or not by comparing the cumulative addition signal of the first field with the cumulative addition signal of the second field and outputs a duplicate field discriminating signal 205 and a threshold value decision signal 206.
The decision is made as follows. The result of the cumulative addition of the first field is multiplied with a coefficient greater than 1 and compared with the result of the cumulative addition of the second field. When the result of comparison is smaller than a predetermined value, it is decided that the first field may be a duplicate field. The result of the cumulative addition of the second field is multiplied with a coefficient greater than 1 and compared with the result of cumulative addition of the first field. When the result of comparison is smaller than a predetermined value, it is decided that the second field may be a duplicate field. In such case, the duplicate field discriminating signal 205 is turned ON. It is also decided that when each of the above-mentioned results of comparison is greater than the predetermined value, the fields are not duplicated fields and the duplicate field discriminating signal 205 is turned OFF.
The threshold value determining means 18 decides a threshold value in correspondence to the threshold value decision signal 206 and outputs a threshold value signal 207. The duplicate field determining means 15 compares the cumulative addition signal 203 with the threshold value signal 207 and decides that the signal is a duplicate field when the cumulative addition signal 203 is smaller than the threshold value signal 207 and the duplicate field discriminating signal 205 is turned ON to output the duplicate field discriminating signal 103. Such arrangement allows the duplicate field to be detected without being affected by noise because the threshold value signal 207 is controlled in correspondence to an amount of conversion noise generated in converting the movie film signal into the telecinema signal and because whether or not the signal is a duplicate field depends on this threshold value signal 207 and the duplicate field discriminating signal 205.
However, the prior art television signal converter, constructed as described above, has had a problem because no large difference is produced between the cumulative value of the difference signal of the first field and the cumulative value of the difference signal of the second field. Namely, when the motion of a scene of the movie film is temporarily small or when a still scene appears, the ratio, even when found, becomes close to 1. Therefore, the input signal erroneously may be recognized as a duplicate field of the telecinema signal.
Further, because many of the movie image signals are displayed lengthily in the horizontal direction, a process called trimming, for inserting black parts at the upper and lower parts of the image, is often implemented in displaying an NTSC signal whose aspect ratio is 4:3 on a screen. Because the difference signal between the fields is always zero when the effect of noise is removed in such black parts, erroneous detection may be a problem when the intra-field difference signals are accumulated for the whole screen.
Moreover, the possibility of picture disturbance caused by the erroneous recognition increases when an input signal of an NTSC signal of 30 Hz is erroneously recognized as a telecinema signal of 30 Hz in processing by the duplicate field removing means. This problem will be explained below with reference to FIG. 16.
For example, when the NTSC signal of 30 Hz as shown in the upper stage of FIG. 16 is input, it is converted into a signal of 24 Hz as shown in the middle stage because it is erroneously detected as a telecinema signal. Then, it is converted into an NTSC signal of 30 Hz in the lower stage by the 3:2 pull-down process after the coding and decoding processes. When Field 603 is removed by the duplicate field removing means for example, the same signal as that of Field 801 is displayed in Field 803 even though the original NTSC signal of Field 603 is different from that of Field 601. When this signal is displayed in a still picture mode in units of frames and Field 803 is displayed in the same time interval as Field 804 for example, a time difference of {fraction (3/60)} seconds is produced between these two fields because the same signal is displayed in Field 801 and in Field 803. That is, a signal having the time difference of {fraction (3/60)} seconds like that between Field 601 and 604, not the time difference of {fraction (1/60)} second like that between Field 603 and Field 604 of the original NTSC signal, is displayed in the same time interval. Thus, a great discrepancy occurs per line of an outline of a moving object in particular, causing a great visual disturbance.
Accordingly, it is an object of the present invention to eliminate the aforementioned problems by detecting a duplicate field of a telecinema signal or the like more accurately.
It is another object of the present invention to reproduce an image having less visual disturbance even when an NTSC signal is erroneously detected as a telecinema signal.
It is an object of the invention to provide an image signal processor which achieves an improved accuracy of detecting a duplicate field.
Another object of the invention is to provide an image signal processor having less visual disturbance even if the image signal is erroneously recognized as the telecinema signal.
According to the present invention, an image signal processor has a duplicate field detecting means for detecting a duplicate field of an image signal based on a threshold value for detecting a duplicate field and a threshold value for detecting a non-duplicate field. The duplicate field detecting means detects a duplicate field by using a threshold value for detecting duplicate field, and detects a non-duplicate field by using a threshold value for detecting a non-duplicate field in order to improve accuracy of detecting a duplicate field.
In one embodiment of the invention, the image signal processor further comprises an improved duplicate field removing means which selects a field which is to be removed based on whether the duplicate field belongs to a first field or second field. The duplicate field removing means removes one of the first fields, which temporally antecedes, when said duplicate field belongs to said first field, and removes said duplicate field detected by the duplicate field detecting means when the duplicate field belongs to the second field.
The specific nature of the invention, as well as other objects, uses and advantages thereof, will clearly appear from the following description and from the accompanying drawings which are given by illustration only, and thus are not limitative of the present invention.